You can use a very simple formula for first order decay...
Fraction Remaining (FR) = 0.5n where n is the number of half lives that have elapsed.
In the current problem we want to find the FR, and we know n = 2 half lives elapsed (60 yr/30yr = 2)
FR = 0.52 = 0.25
Since we started with 5.0 g, and we have 0.25 (1/4) left, that would be 5.0 g x 0.25 = 1.25 g
